Devices for collecting diffuse electromagnetic radiation by the utilisation of fluorescence have been disclosed. The core piece of the collector is a medium which has a higher optical density than the surroundings and which contains centres capable of fluorescence (DE-OS (German Published Specification) No. 2,620,115).
The economic practicability of a light-collecting system is to a large extent determined by the suitability of the dyestuff employed therein as a light converter. Extreme demands are made with regard to the optical quality of this dyestuff [Appl. Phys. 14 123-139 (1977)].